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Nuclear Science User Facilities 102 Post-Irradiation Examination of ATR-Irradiated Ultra-Fine Grained Steel K. Linga Murty North Carolina State University murtyncsu.edu Currently 436 nuclear reactors currently provide an average of about 20 of the energy in the 30 countries that have nuclear power programs 1. However the structural materials that will be used for the new advanced reactor systems will be subjected to far higher neutron fluences 150 dpa than those in todays oper- ating reactors 50 dpa 2. Therefore it is imperative that we develop new more-radiation-tolerant materials and investigate the effects irradiation has on them. Ultra-fine grained UFG materials have shown promising mechanical properties and extensive research has been conducted on the different processing techniques that could improve their properties across various applications 3. Previous studies have shown that the high densities of grain boundaries in UFG materials act as sinks for irradi- ation-induced defects reducing the influence of the irradiation defects 4. Project Description In this study researchers investigated the effects of neutron irradiation on UFG ferritic steel that had been prepared using the equal-channel- angular-pressing ECAP technique 3 4.The experimental material is an ECAPed low-carbon mild steel consisting by weight of 0.1 carbon C 0.5 manganese Mn 0.27 silicon Si and the balance 99.13 iron Fe. Both UFG samples along with their conventional grain CG steel counterpartsprepared by annealing UFG samples at 800C for 1 hourwere irradiated to 1.37 displacements per atom dpa in ATR at INL.The irradiation capsules were designed to keep the irradiation temperature of the samples below 100C.The mean grain sizes for the UFG and CG steels are 0.35 m 0.18 m and 4.4 m 1.8 m respectively Figure 1. Accomplishments After irradiation microstructural and mechanical properties of the CG and UFG steels were investigated using transmission electron microscopy TEM electron back scattered diffrac- tion EBSD atom probe tomography APT x-ray diffraction XRD and micro hardness and tensile testing. TEM micrographs showed no grain growth post irradiation in the UFG steel. ESBD showed similar results for CG steel. XRD was used to determine the dislocation density for both steels before and after irradiation by fitting the XRD patterns to a pseudo-Voigt pV function using the Modified Rietveld technique 5. CG steel exhibited an increase in the dislocation density post irradiation. On the other hand with values well within error bars UFG steel showed no significant change.APT analysis revealed a high number of nano Mn-Si-enriched precipitates in both CG and UFG steels Figure 2. However Figure 1. Pre-irradiation EBSD patterns and grain size distributions for a UFG and b CG steels.